When Trip suffers a catastrophic injury, his only hope for survival is a transplant from a "mimetic simbiot" which Phlox grows from one of his exotic creatures. The simbiot would have Trip's genetic make-up, but it would grow old and die in a span of 15 days. Archer is uneasy about this plan, but he eventually decides that he has no choice: he must complete his mission and he needs Trip to do so.

Connor Trinneer (Adult Sim-Trip)

Adam Taylor Gordon (Sim-Trip, Age 8)
Shane Sweet (Sim-Trip, Age 17)
Maximillian Orion Kesmodel (Sim-Trip, Age 4)

Porthos

• This is the first episode written by co-executive producer Manny Coto, who will go on to executive produce Season 4.
  
• This episode was voted the third most popular episode by fans during the push to the Enterprise series finale in 2005.
  
• Adam Taylor Gordon also played a younger Trip in the dream sequence in "The Xindi".
  
• The adult Sim-Trip confesses to T'Pol he has feelings for her, but he isn't sure if they are his or Trip's.